Smoothing method for layered deposition modeling

ABSTRACT

Disclosed is a method for smoothing the surface of an object built from a polymeric or wax material using a layered manufacturing rapid prototyping technique. After the object is built it is exposed to a vaporized solvent such as in a vaporizer for an exposure time sufficient to reflow the object surface. A solvent is chosen based on its ability to transiently soften the material which forms the object, and thereafter evaporate off the object. The object is removed from the solvent and allowed to dry, producing a smooth finished part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States national phase entry of PCTInternational Application No. PCT/US03/10220, filed on Apr. 4, 2003,which claims the benefit of U.S. Provisional Application No. 60/373,186,filed Apr. 17, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to the field of rapid prototyping, andparticularly to methods of achieving surface smoothness in prototypeobjects made by layered manufacturing.

Production and testing of prototype objects is a commonly used step indeveloping new products, machines and processes in a wide range ofindustries. A variety of layered manufacturing methods for formingthree-dimensional prototypes are known, which develop prototype objectscheaply and quickly from a geometric computer model under computercontrol. These rapid prototyping methods generally slice or divide adigital representation of a desired object (computer aided design (CAD))into horizontal layers, then build the object layer-by-layer byrepetitive application of materials. Exemplary rapid prototypingtechniques include layered deposition modeling, selective lasersintering and stereolithographic processes.

One example of layered deposition modeling is a fused depositionmodeling technique performed by Stratasys® FDM® modeling machines. Fuseddeposition modeling builds up three-dimensional objects by extrudingsolidifiable modeling material from an extrusion head in a predeterminedpattern, layer-by-layer, based upon design data corresponding to theparticular shape of each object layer. Examples of extrusion-basedapparatus and methods for making three-dimensional objects are describedin Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433,Danforth et al. U.S. Pat. No. 5,738,817, Batchelder et al. U.S. Pat. No.5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, all of which areassigned to Stratasys, Inc., the assignee of the present invention.

In the Stratasys® FDM® modeling machines of the current art, modelingmaterial is typically loaded into the machine as a flexible filamentwound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. Asolidifiable material which adheres to the previous layer with anadequate bond upon solidification and which can be supplied as aflexible filament is used as the modeling material Motor-driven feedrollers advance the strand of filament into a liquifier carried by anextrusion head. Inside the liquifier, the filament is heated to aflowable temperature. Flowable modeling material is forced out of anozzle on the far end of the liquifier, and deposited from the liquifieronto a base. The flow rate of the material extruded from the nozzle is afunction of the rate at which the filament is advanced to the extrusionhead. A controller controls movement of the extrusion head in ahorizontal x, y plane, controls movement of the base in a verticalz-direction, and controls the rate at which the feed rollers advancefilament. By controlling these processing variables in synchrony, themodeling material is deposited in “beads” layer-by-layer along toolpaths defined from the CAD model. The material being extruded fuses topreviously deposited material and solidifies to form a three-dimensionalobject resembling the CAD model.

The surfaces of objects developed from layered manufacturing techniquesof the current art are textured or striated due to their layeredformation. Curved and angled surfaces generally have a “stair step”appearance, caused by layering of cross-sectional shapes which havesquare edge profiles. The stair-stepping effect is more pronounced aslayer thickness increases. Although the stair-stepping does not effectthe strength of the object, it does detract aesthetically.

Surface roughness of objects created by layered manufacturing techniquesalso arises from errors in the build process. For example, in the fuseddeposition modeling systems of the current art, errors can arise due inpart to inconsistent extrusion flow rates. Errors particularly occur atstart points and end points of the tool path, for instance, at thelocation of a “seam” (i.e., the start and end point of a closed-looptool path). These errors can cause undesired inconsistencies in theshape of the resulting model.

Rapid prototyping of three-dimensional objects includes not only theproduction of prototype parts, but also may include the production ofmolds. Exemplary uses of molds created with rapid prototyping includeforming molds used to create injection molding tool inserts such asdescribed in U.S. Pat. No. 5,189,781, and forming fugitive molds forgreen ceramic pieces prior to sintering such as described in U.S. Pat.Nos. 5,824,250 and 5,976,457.

The current art teaches manually trimming, machining or grinding objectsformed by layered manufacturing to remove excess material. Buffing withcloths, sand paper or solution-born abrasives are current methods ofsmoothing or polishing the objects. For example, Hull et al. U.S. Pat.No. 5,059,359, Methods and Apparatus for Producing Three-dimensionalObjects by Stereolithography, describes their prototypes as “perfect forsmoothing by sanding to yield the right-sized part”. The need forhand-finishing of models made from additive process techniques is alsorecognized in U.S. Pat. No. 6,021,358, which utilizes subtractivemodeling techniques to attain smooth models. There is a need in rapidprototyping systems of an expedient and relatively inexpensive method ofpost-processing layered manufacturing prototype objects.

A previously developed technique used in manufacturing of plasticsinvolves the use of chemical vapors or liquids to smooth by reflowingthe surface of the plastic, termed solvent polishing. Solvent polishingwas developed in the plastics industry over twenty years ago for thepurpose of developing a smooth level and/or high gloss coating orsurface without needing to exercise extreme care in the application ormanufacturing of the items. For example, U.S. Pat. No. 3,437,727discloses a method using chemical vapors for refinishing telephones thatwere returned to the telephone company as a method of recycling them.

There are two standard methods for solvent polishing articles. The firstis to immerse the entire plastic article in a bath of liquid plasticsolvent for a period of time based on the solvent and type of plasticinvolved. This allows the solvent to penetrate the outer layer of theplastic, thereby causing it to reflow. Reflowing causes the outersurfaces of the plastic article to become smooth and/or shiny.

The second method of solvent polishing is the exposure of the plasticarticle to vaporized solvent. A handheld vaporizer as shown in U.S. Pat.No. 4,260,873 may be used to expose the plastic object. Alternatively,the object can be placed into a chamber filled with a vaporized solvent,generated from a heated bath below, as in U.S. Pat. No. 3,737,499. Anadvantage of the vaporizing chamber is that the solvent is contained andcan be recycled, thereby minimizing potential environmental pollution.

The use of hot solvent vapors to melt or plasticize the surface of thesubstrate has been used in the circuit board manufacturing area tofacilitate the transfer of printed circuits, as disclosed, for example,in U.S. Pat. No. 4,976,813. Another example is disclosed in U.S. Pat.No. 4,594,311, where solvent vapor is used to treat the non-imaged areasof the plastic base material which holds a printed circuit board inorder to further enhance the printed pattern and secure it more stronglyto the surface. In U.S. Pat. No. 5,045,141, a substrate, typically acircuit board, may be treated to facilitate transfer of the printedcircuit to it.

Solvent polishing using liquid or vapors is also commonly used as adegreasing or cleaning step in manufacturing processes, especially priorto painting.

Despite the need in rapid prototyping for an expedient and inexpensivesurface finishing technique, Applicant is unaware of any teaching orsuggestion in the prior art to use a vapor polishing technique for thesmoothing of objects formed by layered manufacturing rapid prototypingtechniques.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method for smoothing the surface of an objectbuilt from a polymeric or wax material using a layered manufacturingrapid prototyping technique. After the object is built, it is exposed toa vaporized solvent for an exposure time sufficient to reflow the objectsurface. A solvent is chosen based on its ability to transiently softenthe material which forms the object, and thereafter evaporate off theobject. The object is removed from the solvent and allowed to dry,producing a smooth finished part. Optionally, portions of the objectsurface may be masked prior to exposing the object to solvent, so as topreserve fine details of the object. Alternatively, portions of theobject surface may be pre-distorted prior to exposing the object tosolvent, so that said surface portions will attain a desired geometryfollowing vapor smoothing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, magnified view illustrating a raw object formedby a layered manufacturing rapid prototyping technique.

FIG. 2 is a perspective, magnified view of the object shown in FIG. 1,after undergoing vapor smoothing.

FIG. 3 is a diagrammatic view illustrating the process of vaporsmoothing an object in accordance with the present invention.

FIG. 4 a is a cross-sectional detailed view of a portion of a raw objectformed by fused deposition modeling.

FIG. 4 b shows the object cross-section of FIG. 4 a after vaporsmoothing.

FIG. 5 a is a cross-sectional detailed view of the object portion shownin FIG. 4 a, wherein the object geometry has been pre-distorted inanticipation of vapor smoothing.

FIG. 5 b shows the object cross-section of FIG. 5 a after vaporsmoothing.

DETAILED DESCRIPTION

The method of the present invention may be employed with respect toobjects formed from a polymeric or wax material using layeredmanufacturing rapid prototyping techniques. An exemplary layeredmanufacturing technique is the type disclosed in U.S. Pat. No.5,121,329, wherein an extrusion head deposits “roads” of molten materialin layers of predetermined shape, and which material solidifies upon adrop in temperature to form a solid model.

FIG. 1 shows an exemplary object 10, formed by a layered manufacturingrapid prototyping technique. The object 10 has an angled surface 12, acurved surface 14, two horizontal surfaces 16, and three verticalsurfaces 18. In another embodiment, the object may be a mold tool foruse in making prototype plastic injection molded parts, such as isdisclosed in International Application No. PCT/US03/010219 entitled“Layered Deposition Bridge Tooling”, S. Crump and J. Hanson, filed oneven date herewith and assigned to the same assignee as the presentapplication. The object 10 is made of a polymeric or wax modelingmaterial, which may also include fillers and other additives as well asdispersed particulate materials. Amorphous thermoplastics areparticularly suited for use in the present invention, for instance, ABS,polycarbonate, polyphenylsulfone, polysulfone, polystyrene,polyphenylene ether, amorphous polyamides, acrylics,poly(2-ethyl-2-oxazoline), and blends thereof. The present invention mayalso be used to advantage with other polymeric and wax materials,including glass-filled nylon, jetting wax, sintered thermal plasticpowders, and green metals or green ceramics dispersed in a polymericbinder.

As shown in FIG. 1, the object 10 is “raw”, that is, it has notundergone post-process smoothing. Prior to vapor smoothing in accordancewith the present invention, surfaces 12 and 14 exhibit a stair-steppingeffect. Surfaces 16 and 18 exhibit striation and roughness.Additionally, object 10 exhibits porosity due to the fused depositionmodeling technique.

To smooth the surfaces of object 10, the object 10 is placed in avaporizer 30, where it is exposed to vapors of a solvent 34. This isillustrated in FIG. 3. A suitable vaporizer is of the type availablefrom Detrex Corp. of Southfield, Mich., model VS-2000, although thoseskilled in the art will recognize that many alternative vaporizers canbe used in practicing the present invention. Vaporizer 30 is shown ashaving a control panel 31 for controlling operation of the vaporizer,and primary and secondary cooling coils, 33 and 35, respectively.

The solvent 34 is selected to be compatible with the modeling materialwhich forms the object 10. Suitable solvents will react with themodeling material so as to soften and flow the material at the objectsurfaces. The solvent should also be able to vaporize off the surface ofthe object, leaving the object intact and unscathed. A preferred solventfor use with a wide range of amorphous thermoplastics is methylenechloride. Other suitable solvents will be recognized by those skilled inthe art, for instance, an n-Propyl bromide solution (e.g., Abzol®),perchloroethylene, trichloroethylene, and a hydrofluorocarbon fluid soldunder the name Vertrel®.

As illustrated in FIG. 3, the vaporizer 30 boils the solvent 34 into avapor zone 36, which is maintained at or above the boiling point of thesolvent and contained by the cooling coils 33 and 35. The object 10 issuspended in the vapor zone 36, held by a wire skewer 32, which is bentto fit around the object. Alternative holding means may also be used,such as a basket, a net or a mesh platform. The object 10 is exposed tothe vaporized solvent 34, allowing vapors of the solvent 34 to penetratethe surfaces 12, 14, 16 and 18 of object 10. Penetration of the solvent34 softens the modeling material at the object surfaces, so that thesurface material may reflow. Reflowing of the material smoothes theobject surfaces.

The object 10 remains exposed to the vapors of solvent 34 until adesired surface finish is obtained. An exposure time is selected basedon the type of solvent and modeling material, the fineness of the objectfeatures, and the concentration of the solvent vapors. The exposure timecan be gauged by observing condensation of solvent vapors on the object,or can be pre-selected according to a formula. Condensation will stopwhen the temperature of the object surface reaches the temperature ofthe boiling solvent. This is an indication that solvent penetration hasoccurred. Using methylene chloride as the solvent 34, an amorphousthermoplastic modeling material will soften and reflow in a short time,typically between about 0.1 seconds to 5 minutes exposure time. Ifsmoothing of an object is expected to occur in a short exposure time, itmay be desirable to reduce the concentration of solvent vapors so thatthe exposure time can be increased. A longer exposure time allows anoperator more time to observe the smoothing process and more room forerror in removing the object from the solvent vapors.

When the exposure time elapses, the object 10 is removed from the vaporzone 36 and allowed to dry. In a preferred embodiment, the object 10dries within seconds to minutes after its removal from the vapor zone36, as the solvent 34 evaporates off of the object 10. The object 10 maythen be handled, as it is not sticky, soft or wet. In some cases, suchas where solvent exposure time is great or the solvent is highlyconcentrated, it may be desirable to dry the object 10 in an oven toremove any residual solvent. Oven drying should be done at a temperaturegreater than the boiling point of the solvent.

Following the vapor smoothing process, the stair steps in surfaces 12,14, 16 and 18 of object 10 are either significantly reduced oreliminated. The extent of the smoothing achieved for a given objectusing the method of the present invention will depending upon theexposure time, the solvent, the modeling material, and the initialsurface condition of the object. FIG. 2 illustrates a significantreduction in the stair steps and roughness of the object 10, achieved byvapor smoothing. Following the vapor smoothing process, the porosity ofobject 10 in surfaces 12, 14, 16 and 18 is also either reduce oreliminated.

Optionally, selected features of an object (e.g., features smaller than0.25 inches, thin walls, corners, convex edges and concave edges) can bemasked with a substance that will inhibit smoothing of said selectedportions, or, exposure of said selected features to the solvent vaporscan be otherwise avoided. For example, it may be desirable to mask thecorners of object 10, to prevent the corners from rounding. Similarly,concave surfaces of an object can be masked to prevent in-flow ofsurrounding material. Suitable solvent masking substances include thoseused in printed circuit board manufacturing, such as gums, waxes,pastes, adhesives or masking tape, which may be applied either manuallyor automatically. Masking may also be accomplished by surrounding afeature with a gas.

Automatic application of a masking substrate may be done, for example,by jetting a masking material onto the surface of selected objectfeatures, in a layered deposition process such as is known in the art. Amasking substance may also be applied by depositing roads of maskingmaterial, using a fused deposition modeling process such as performed byStratasys® FDM® modeling machines. Those skilled in the art willrecognize additional masking techniques know in the art, that may beapplied in carrying out the present invention.

When an automatic masking technique is used, the features to be maskedmay be identified using a software algorithm that creates a digitalrepresentation of the surface area to be protected. The protected areamay be identified in a digital representation of the object, such as inan .stl file geometry using a CAD system, a Graphic Pixel system or aVoxel system. Alternatively, the surface areas to be masked may beidentified by the user via a haptic input interface, such as a FreeForm™system available from SensAble Technologies, Inc. The haptic inputsystem creates a digital mask of the areas for which smoothing is notdesired.

As an alternative to masking techniques, the geometry of an objectsurface may be pre-distorted in anticipation of the vapor smoothing. Thepre-distortion is accomplished by using a software algorithm to modify adigital representation of the object (e.g., a CAD model of the object ora sliced representation of the object as in a .stl file). Using apre-distortion software algorithm, feature details are distorted so asto overbuild corners and edges, and underbuild pockets, such thatfollowing vapor smoothing such features will attain approximately thedesired geometry. More specifically, an exemplary pre-distortionalgorithm will: (1) identify geometric features with radii of curvatureequal to or smaller than the slice height (i.e., the thickness of alayer); (2) for identified features having a positive radius ofcurvature (e.g., a corner or edge), the algorithm will build up theinitial object representation at such features; and (3) for identifiedgeometric features having a negative radius of curvature (e.g., a pocketor a joint between planes), the algorithm will hollow out the objectrepresentation in the vicinity of such features. The pre-distortionsoftware algorithm thus creates a modified object representation, sothat the identified geometric features will be distorted by eitherdepositing additional material or depositing less material than isultimately desired in the final smoothed object. A similar algorithm canbe used to identify features for masking.

According to the pre-distortion algorithm, features should be built upby not more than the slice height, for instance, by half of a sliceheight. The surface roughness of a typical part made by fused depositionmodeling is about 0.3 times the slice height. The additional materialadded in pre-distortion of positive features may be roughly thethickness of this surface roughness, so that when the reflowed materialis pulled away, the solid material left takes on the desired finalobject geometry. For the negative curvature regions, enough materialneeds to be removed by the pre-distortion algorithm that the in-flowfrom the surrounding regions fills in the removed material.

Pre-distortion of object geometry is illustrated in FIGS. 4 a and 4 band FIGS. 5 a and 5 b. FIGS. 4 a and 4 b show a cross-sectional view ofa portion of an object 40 that has not been pre-distorted, superimposedonto an outline 42 illustrating the desired final surface objectgeometry of object 40 (i.e., the unmodified object representation). Asillustrated in FIG. 4 b, vapor smoothing results in rounding of convexcorners 44 away from the desired outline 42, and rounding of edges 46beyond the desired outline 42. FIGS. 5 a and 5 b illustrate a portion ofan object 40′ which has the same desired final surface geometry asobject 40. Unlike object 40, object 40′ has been pre-distorted accordingto the pre-distortion algorithm of the present invention. As illustratedin FIG. 5 a, the pre-distorted surface geometry of object 40 extendsbeyond the desired outline 42 at corners 44 and concave edges 46.Following vapor smoothing, as illustrated in FIG. 5 b, the corners 44and edges 46 of the pre-distorted object 40 more closely follow thedesired outline 42 than do the corners 44 and edges 46 of the object 40.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

The invention claimed is:
 1. A method for making a three-dimensionalobject comprising the steps of: providing an object built from amodeling material using a fused deposition modeling technique, whereinthe modeling material comprises a thermoplastic resin, wherein the builtobject has an object surface formed of the modeling material, the objectsurface having at least one surface effect due to the fused depositionmodeling technique that extends substantially across an entirety of theobject surface, wherein the at least one surface effect comprises astair step effect, striation, or a combination thereof, and wherein theobject exhibits porosity due to the fused deposition modeling technique;placing the object in a vessel; exposing substantially the entire objectsurface to vapors of a solvent that transiently softens the modelingmaterial at the object surface while the object resides in the vessel ina solvent vapor zone present below cooling elements in the vessel;reflowing the softened modeling material to reduce the at least onesurface effect and to reduce the porosity of the object at the objectsurface; observing condensation of the solvent vapors on the objectsurface while reflowing the softened modeling material; discontinuingthe exposure of the object surface to the solvent vapors after thecondensation of the solvent vapors stops; and drying the object afterdiscontinuing the exposure, wherein the object surface of the driedobject is substantially free of the at least one surface effect and issubstantially free of the porosity.
 2. The method of claim 1, whereinthe thermoplastic resin comprises at least about 50 weight percent of anamorphous thermoplastic selected from the group consisting of ABS,polycarbonate, polyphenylsulfone, polysulfone, polystyrene,polyphenylene ether, amorphous polyamides, acrylics,poly(2-ethyl-2-oxazoline), and blends thereof.
 3. The method of claim 2,wherein the solvent is selected from the group consisting of methylenechloride, an n-Propyl bromide solution, perchloroethylene,trichloroethylene, and a hydrofluorocarbon fluid.
 4. The method of claim1, and further comprising the step of: masking selected portions of theobject surface with a substance that will inhibit smoothing of theselected portions, prior to the step of exposing the object to thevapors of the solvent.
 5. The method of claim 1, and further comprisingbuilding the object using the fused deposition modeling technique. 6.The method of claim 1, and further comprising the step of: suspendingthe object in the vessel containing the vapors of the solvent in amanner that substantially allows the entirety of the object surface tobe exposed to the vapors of the solvent.
 7. The method of claim 1, andfurther comprising the steps of: providing an initial objectrepresentation in a digital format, the initial object representationhaving a surface geometry; and modifying the initial objectrepresentation to pre-distort certain features of the surface geometry,producing a modified object representation; wherein the object built inthe building step has a geometry defined according to the modifiedobject representation; and wherein the desired geometry attainedfollowing the exposing step approximately matches that of the initialobject representation.
 8. A method for making a three-dimensional objectcomprising the steps of: providing an object built from a plurality oflayers with a modeling material using a fused deposition modelingtechnique, wherein the modeling material comprises a thermoplasticresin, wherein the object has an object surface, and wherein theplurality of layers create at least one surface effect extendingsubstantially across an entirety of the object surface, wherein the atleast one surface effect comprises a stair step effect, striation, or acombination thereof, and wherein the object exhibits porosity due to thefused deposition modeling technique; placing the object in a vessel;exposing substantially the entire object surface to vapors of a solventwhile the object resides in the vessel in a solvent vapor zone presentbelow cooling elements in the vessel; penetrating the object surfacewith the vapors of the solvent to soften the modeling material at theobject surface; reflowing the softened modeling material to reduce theat least one surface effect substantially across the entirety of theobject surface and to reduce the porosity of the object at the objectsurface; observing condensation of the solvent vapors on the objectsurface while reflowing the softened modeling material; discontinuingthe exposure of the object surface to the solvent vapors after thecondensation of the solvent vapors stops; and drying the object afterdiscontinuing the exposure, wherein the object surface of the driedobject is substantially free of the at least one surface effect and issubstantially free of the porosity.
 9. The method of claim 8, whereinthe thermoplastic resin comprises at least about 50 weight percent of anamorphous thermoplastic selected from the group consisting of ABS,polycarbonate, polyphenylsulfone, polysulfone, polystyrene,polyphenylene ether, amorphous polyamide, methyl methacrylate,poly(2-ethyl-2-oxazoline), and blends thereof.
 10. The method of claim8, and further comprising the step of: masking selected portions of theobject surface with a substance that will inhibit smoothing of theselected portions, prior to the step of reflowing the surface.
 11. Themethod of claim 10, wherein the masking substance is applied using anautomatic process selected from the group consisting of a jettingprocess and a fused deposition modeling process.
 12. The method of claim8, wherein the solvent is selected from the group consisting ofmethylene chloride, an n-Propyl bromide solution, perchloroethylene,trichloroethylene, a hydrofluorocarbon fluid, and combinations thereof.13. A method for treating a three-dimensional object, the methodcomprising: providing the three-dimensional object, thethree-dimensional object being previously built from a modeling materialcomprising a thermoplastic resin using a fused deposition modelingtechnique, wherein the three-dimensional object comprises an exteriorsurface having at least one surface effect caused by the fuseddeposition modeling technique that extends substantially across anentirety of the exterior surface, wherein the at least one surfaceeffect comprises a stair-step effect created by layering of a pluralityof layers of the modeling material, striation created by formation ofroads of the modeling material, or a combination thereof, and whereinthe three-dimensional object exhibits porosity due to the fuseddeposition modeling technique; placing the three-dimensional object in avessel configured to contain solvent vapors; while the three-dimensionalobject resides in the vessel, exposing substantially the entire exteriorsurface of the three-dimensional object while the object is in a solventvapor zone present below cooling elements in the vessel to the solventvapors, wherein the solvent vapors transiently soften the modelingmaterial across the entire exposed exterior surface of thethree-dimensional object; observing condensation of the solvent vaporson the exterior surface of the three-dimensional object while theexterior surface of the three-dimensional object is exposed to thesolvent vapors; removing the three-dimensional object from the vesselafter the condensation of the solvent vapors on the exterior surface ofthe three-dimensional object stops; and drying the three-dimensionalobject after removing the three-dimensional object from the vessel,wherein the entire exposed exterior surface of the driedthree-dimensional object is substantially free of the at least onesurface effect and is substantially free of the porosity.
 14. The methodof claim 13, wherein the vessel is configured to be maintained at orabove a boiling point of a solvent of the solvent vapors.
 15. The methodof claim 13, wherein the thermoplastic resin is selected from the groupconsisting of ABS, polycarbonate, polyphenylsulfone, polysulfone,polystyrene, polyphenylene ether, amorphous polyamides, acrylics,poly(2-ethyl-2-oxazoline), and blends thereof.
 16. The method of claim13, wherein the solvent is selected from the group consisting ofmethylene chloride, an n-Propyl bromide solution, perchloroethylene,trichloroethylene, a hydrofluorocarbon fluid, and combinations thereof.17. The method of claim 13, wherein placing the three-dimensional objectin the vessel comprises suspending the three-dimensional object in thevessel.
 18. The method of claim 13, and further comprising maskingselected portions of the exterior surface with a substance that willinhibit smoothing of the selected portions.